For example, Japanese Patent Application Publication No. 2009-020304 discloses an autofocus method in which an image signal is obtained by projecting an image of a subject which is picked up by an imaging lens onto an imaging device and a focusing point of the subject is obtained by controlling a focus position of the imaging lens. In this method, it is determined whether or not the subject is backlighted, and based on the result, an autofocus detection area for the subject is switched between a backlight state and a non-backlight state. In the backlight state, a focusing position of the subject is detected from an edge of the subject in the detection area in an output signal of the imaging device.
For surveillance purposes, a backlight scene may be generated in a dark place such as nighttime or the like. For example, spot light may be directly applied by a strong light source such as illumination light or the like to the vicinity of a subject to be focused. In a general contrast detection autofocus method, a focus lens is moved to a position where a high frequency component (differential component) of an image signal becomes maximum.
However, in a backlight scene in a dark place, a contrast difference between a high luminance (highlight) region such as spot light generated by illumination light and a low luminance region such as a surrounding dark part becomes relatively higher than a contrast difference of contour components of the subject seen at the time of focusing. Therefore, particularly if a differential signal is used for a focus signal (AF (Automatic Focus) evaluation value) in a scene having relatively few differential signal components from the subject, the influence of the spot light becomes stronger, which makes normal operation of autofocus difficult. In other words, the spot light of the illumination device spreads as it is deviated from the in-focus position. Accordingly, the contour also becomes greater, and the focus signal becomes greater.
Thus, it is required to distinguish a differential signal from a boundary between a high luminance region and a low luminance region due to illumination light or the like and a differential signal of a contour component of a subject seen at the time of focusing in a backlight scene in a dark place and determine whether or not it is added as an AF evaluation value.